ARM: dts: socfpga: Fix pca9548 i2c-mux node name

"make dtbs_check":

    arch/arm/boot/dts/socfpga_cyclone5_vining_fpga.dtb: i2cswitch@70: $nodename:0: 'i2cswitch@70' does not match '^(i2c-?)?mux'
	    From schema: Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.yaml
    arch/arm/boot/dts/socfpga_cyclone5_vining_fpga.dtb: i2cswitch@70: Unevaluated properties are not allowed ('#address-cells', '#size-cells', 'i2c@0', 'i2c@1', 'i2c@2', 'i2c@3', 'i2c@4', 'i2c@5', 'i2c@6', 'i2c@7' were unexpected)
        From schema: Documentation/devicetree/bindings/i2c/i2c-mux-pca954x.yaml

Fix this by renaming the PCA9548 node to "i2c-mux", to match the I2C bus
multiplexer/switch DT bindings and the Generic Names Recommendation in
the Devicetree Specification.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>